docs: silence the 7 DocumenterInterLinks local-inventory warnings in CI

[ocots]

Symptom

Every Documentation.yml run (CI, not just the new GPU pass — this predates #885) logs 7 warnings like:

┌ Warning: Failed to load inventory "CTBase" from possible source "/home/runner/work/OptimalControl.jl/OptimalControl.jl/docs/../../CTBase/docs/build/1/objects.inv".
│   exception = SystemError: opening file "...CTBase/docs/build/1/objects.inv": No such file or directory
└ @ DocumenterInterLinks ~/.julia/packages/DocumenterInterLinks/U7nII/src/interlinks.jl:233

One per control-toolbox sibling package (CTBase, CTDirect, CTFlows, CTLie, CTModels, CTParser, CTSolvers).

Why it happens

docs/make.jl:161-222 builds each InterLinks entry as an ordered tuple of sources, e.g.:

"CTBase" => (
    "https://control-toolbox.org/CTBase.jl/stable/",
    joinpath(@__DIR__, "..", "..", "CTBase", "docs", "build", "1", "objects.inv"),
    "https://control-toolbox.org/CTBase.jl/stable/objects.inv",
),

DocumenterInterLinks tries sources in order and uses the first that loads. The local objects.inv path only exists for a contributor who has cloned the sibling repos side-by-side and built their docs locally — deliberately kept first so that cross-repo docstring/@extref changes can be verified locally before either side is released (this ordering was reaffirmed in the #885 follow-up discussion: local-first is wanted, not accidental). On the CI runner only OptimalControl.jl is checked out, so that path always fails, DocumenterInterLinks logs a Warning, then falls back to the remote URL, which succeeds. End result is correct; the log is just noisy — 7 warnings on every single run.

The other 4 entries (ADNLPModels, NLPModelsIpopt, ExaModels, MadNLP, Tutorials) use committed docs/inventories/*.toml files as their local source instead of a sibling build path, so they never hit this — only the 7 control-toolbox family entries are affected.

Requested fix

Keep local-first ordering for local dev, but only include the local sibling path when not running in CI, so the CI runner's InterLinks tuple has exactly 2 entries (base URL + remote inventory) and the local path — which can never succeed there — is never attempted:

# CI never has sibling repos checked out next to this one, so the local sibling
# `objects.inv` path can never resolve there — only include it outside CI, so
# DocumenterInterLinks doesn't log 7 "Failed to load inventory" warnings on every run.
# Order stays local-first when present: a contributor with siblings cloned and built
# locally sees their own in-progress docstring/@extref changes, not the last stable
# release.
const IN_CI = get(ENV, "CI", "false") == "true"

sibling_inventory(pkg, base_url) = IN_CI ? (base_url, "$(base_url)objects.inv") : (
    base_url,
    joinpath(@__DIR__, "..", "..", pkg, "docs", "build", "1", "objects.inv"),
    "$(base_url)objects.inv",
)

links = InterLinks(
    "CTBase" => sibling_inventory("CTBase", "https://control-toolbox.org/CTBase.jl/stable/"),
    "CTDirect" => sibling_inventory("CTDirect", "https://control-toolbox.org/CTDirect.jl/stable/"),
    "CTFlows" => sibling_inventory("CTFlows", "https://control-toolbox.org/CTFlows.jl/stable/"),
    "CTLie" => sibling_inventory("CTLie", "https://control-toolbox.org/CTLie.jl/stable/"),
    "CTModels" => sibling_inventory("CTModels", "https://control-toolbox.org/CTModels.jl/stable/"),
    "CTParser" => sibling_inventory("CTParser", "https://control-toolbox.org/CTParser.jl/stable/"),
    "CTSolvers" => sibling_inventory("CTSolvers", "https://control-toolbox.org/CTSolvers.jl/stable/"),
    "ADNLPModels" => (...),  # unchanged, all 4 below unchanged
    ...
)

Note: joinpath(pkg, "docs", "build", "1", ...) isn't uniform across the 7 — CTDirect.jl/CTFlows.jl/CTModels.jl/CTParser.jl carry the .jl suffix in their directory name (CTDirect.jl, not CTDirect) while CTBase/CTLie/CTSolvers don't; sibling_inventory's first argument must be the actual directory name, not the package name — check each existing joinpath(...) call in docs/make.jl when porting rather than assuming they all match the InterLinks key.

Non-goals

• Do not flip the ordering (URL-first): that was considered and rejected — it would make the CI-nominal path depend on network reachability of control-toolbox.org instead of a deterministic local-path failure, to save only cosmetic log noise.
• Low priority / cosmetic only — no functional impact, log noise only.